Non Patent Literature 1 discloses two kinds of methods referred to as a passive scan and an active scan, as a method by which a terminal scans an access point (base station) in a wireless LAN (Local Area Network).
In the passive scan method, a terminal scans presence of an access point, depending on the presence or absence of reception of a beacon frame which is periodically incoming from the access point through an object radio channel to be scanned. In the passive scan method, the terminal needs to wait for the beacon frame to be transmitted from the access point. Therefore, the terminal needs to wait up to periodic time for which the beacon frame is transmitted, in order to confirm the presence or absence of an access point in each radio channel.
On the other hand, in the active scan method, a terminal broadcasts a probe request frame through a radio channel. An access point, which has received the probe request frame, transmits a probe response frame to the terminal. Therefore, the terminal scans presence of the access point, depending on the presence or absence of reception of the probe response frame.
In this way, in the active scan method, the terminal can issue the probe request frame at desired timing, and collect the probe response frame from the access point. Accordingly, the active scan method is generally used in a case where information on the presence of the access point is immediately needed as upon handover or the like.
Hereinafter, operation examples of the active scan will be described with reference to FIGS. 39 and 40.
Firstly, one operation example is described with reference to FIG. 39, in a case where the terminal performs the active scan by using a radio channel in which no access point exists.
When a terminal 20 switches to a channel CH1 at time T101, the terminal 20 waits until ProbeDelay time elapses or a frame is received from a different node. Assume that the frame is not received from the different node.
When the ProbeDelay time elapses at time T102, the terminal 20 starts preparing to transmit a probe request frame FR7c. When transmission of the frame by the different node is not detected from the time T102 to time 1103 after DIFS (Distributed Inter Frame Space) time, the terminal 20 broadcasts the probe request frame FR7c to the channel CH1 at the time T103. At the same time, the terminal 20 starts to time MinChannelTime and MaxChannelTime (not shown). Note that “MinChannelTime<MaxChannelTime” is met, and each of the MinChannelTime and MaxChannelTime is generally set to a value from several to dozens of msec.
In this case, there is no access point in the channel CH1. Therefore, none of access points 10_1 to 10_3 can respond to the probe request frame FR7c transmitted by the terminal 20. When the MinChannelTime elapses at time T104, the terminal 20 assumes that there is no access point in the channel CH I, and thus terminates the active scan, because a response to the probe request frame FR7c could not be received by the MinChannelTime elapsed.
Next, another operation example is described with reference to FIG. 40, in a case where the terminal performs the active scan by using a radio channel in which the access point exists.
Assume that each of access points 10_1 to 10_3 shown in FIG. 40 uses the channel CH1 and exists within a range where the probe request frame FR7c from the terminal 20 can reach. In this case, when the terminal 20 switches to the channel CH1 at time T201, the terminal 20 waits until the ProbeDelay time elapses or the frame is received from the different node. Assume that the frame is not received from the different node.
When the ProbeDelay time elapses at time T202, the terminal 20 starts preparing to transmit the probe request frame FR7c. When the transmission of the frame by the different node is not detected from the time T202 to time T203 after the DIFS time, the terminal 20 broadcasts the probe request frame FR7c to the channel CH1 at the time T203. At the same time, the terminal 20 starts to time the MinChannelTime and MaxChannelTime.
In this case, each of the access points 10_1 to 10_3 receives the probe request frame FR7c and transmits a probe response frame FR8c to the terminal 20. As shown in FIG. 40, assume that the access point 10_1 firstly transmits the probe response frame FR8c. The terminal 20 stops to time the MinChannelTime at time T204 when the probe response frame FR8c is received, and returns an acknowledgment (Ack) frame FR3. Thereafter, when the terminal 20 receives the probe response frames FR8c from the access points 10_2 and 10_3, the terminal 20 returns the Ack frames FR3 to each of the access points 10_2 and 10_3. The MaxChannelTime elapses at time T205. Thus, the terminal 20 recognizes as a result of the scan the access points 10_1 to 10_3 which are transmission sources of the probe response. frames FR8c having been received by then, and terminates the active scan.